# In calculating policy gradients, wouldn't longer trajectories have more weight according to the policy gradient formula?

In Sergey Levine's lecture on policy gradients (berkeley deep rl course), he show that policy gradient can be evaluated according to the formula In this formula, wouldn't longer trajectories get more weight (in finite horizon situations), since the middle term, the sum over log pi, would involve more terms? (Why would it work like that?)

The specific example I have in mind is pacman, longer trajectories would contribute more to the gradient. Should it work like that?

## 1 Answer

wouldn't longer trajectories get more weight?

Not necessarily. Gradient $$\triangledown_{\theta}$$ could be negative or positive (1D analogy), therefore, larger number of gradients could have a smaller weight, which makes sense. A consistent short trajectory is more informative (has more weight) than an inconsistent long trajectory with sign-alternating policy gradients.

Why would it work like that?

If we are comparing two consistent trajectories, where most gradients are in the same direction, this formula makes sense again. A long consistent trajectory contains more useful information (more steps that confirm each other) than a short one. In real life, compare the informativeness of a successful week to a successful year for your policy learning.